This invention relates to a process for making blends of monoolefin rubber and high-diene hydrocarbon rubber and to blends produced thereby.
Monoolefin rubber, typified by terpolymers of ethylene, propylene and a minor portion of diene monomer (EPDM rubber) has particularly good resistance to the degrading effects of oxygen or ozone, among other good properties. In other respects, however, unvulcanized monoolefin rubber has poor tack properties and is consequently unsuited to the production of built-up molded articles such as tires wherein assembly of uncured components requires good tack.
A high-diene hydrocarbon rubber, such as natural and synthetic polyisoprene, polybutadiene and copolymers of butadiene with other monomers such as styrene, has better tack, but is comparatively more susceptible to oxygen and ozone degradation, probably because of the relatively high number of double-bonds in its molecular backbone.
Blends of monoolefin rubber (e.g., EPDM) with high-diene rubber (e.g., natural rubber) appeared to be the answer to the search for a rubbery material which would combine the good properties of each component. Unfortunately, simple blends of these two materials have not proved to be successful, except those in which only a small amount of one or the other type of rubber was present. Thus, if enough EPDM rubber is used to give good ozone resistance, because of fundamental dissimilarities in the two types of rubber, blends of significant amounts of one with another result in heterogeneous mixtures with poor properties. The two types of rubber can be said to be "technologically incompatible," differing in such properties as unvulcanized-state viscosity, surface energy, and vulcanization rate.
The various methods of attempting to resolve this technological incompatability between monoolefin copolymer rubber and high-diene hydrocarbon rubber have all left something to be desired. Such methods include (1) using special vulcanization systems and special accelerators to try to achieve optimum vulcanization of both phases, (2) making EPDM rubber with significantly higher diene content, (3) modifying EPDM rubber, by using a variety of techniques, in an effort to increase its vulcanization rate and (4) prevulcanizing EPDM rubber before blending high-diene hydrocarbon rubber with it.
All of these methods have produced some improvements in the properties of the blends; in most instances the improvements were not sufficient to justify their cost.
Accordingly, a blend of monoolefin copolymer rubber and high-diene rubber which would have the good properties of each component is still unrealized by the industry. One application to which such blends are most suited is in tire sidewalls, where the maximum resistance to oxidation, and especially ozone attack, is needed, and yet good processability and low hysteresis are also prerequisites.